Control system



May 15, 1945. E. E. MOYER CONTROL SYSTEM Filed Dec 11, 1945 TO ID TO IDOF IODE ODE -T0 ANODE OF TRIODE 1 TL1$Z'% Cl T 5| T I 4,5 54 Inventor:

I Elmo E. Meyer,

y His Attorney.

Patented May 15, 1945 CONTROL SYSTEM Elmo E. Moyer, Scotia, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication December 11, 1943, Serial No. 513,898

9 Claims.

This invention relates to control systems, more particularly to controlsystems for electric motors, and it has for an object the provision or asimple, reliable, and inexpensive control system of this character.

More specifically the invention relates to motor control systems inwhich a direct-current motor is supplied through electric valveapparatus from an alternating-current source, and a more specific objectof the invention is the provision in a motor control system of thischaracter of means for accurately maintaining a preset speed of themotor and in reducing to a minimum the apparatus required.

In carrying the invention into effect in one form thereof, a gridcontrolled half-wave rectifier valve is connected between the armatureof a direct-current motor and an alternating-cunrent source so thathalf-wave impulses are supplied to the motor. The amount of currentsupplied by this valve to the motor armature is varied by means of anetwork which controls the grid voltage of the valve. This networkcomprises a capacitor and two parallel and reversely connected electricvalves connected across the alternating-current source. Connections areprovided from the capacitor to the cathode and grid of the valve whichsupplies current to the armature. One of the reversely connected valvesis a grid controlled valve, and variation of its grid voltage in turnvaries the grid voltage of the valve in the armature circuit. A sourceof adjustable reference voltage is provided together with means forderiving from the motor armature a voltage which is dependent upon thespeed of the motor. These two voltages are compared, and the differenceis applied to the grid of the reversely connected control valve, whichthen operates to cause the valve in the armature circuit to cause themotor to operate at a speed corresponding to the magnitude of thereference voltage. Connections between the valve in the armature circuitand the reversely connected control valve prevent the operation of thelatter valve during that portion of the voltage wave in which thearmature valve is conducting.

For a better and more complete understanding of the invention, referenceshould now be had to th following specification and to the accompanyingdrawing of which Fig. 1 is a simple, diagrammatical sketch of anembodiment of the invention; and Figs. 2, 3, 4, 5 and 6 aremodifications.

Referring now to the drawing, the armature I0 of a direct-current motoris connected in series with the anode-cathode conducting path of anelectric valve ll across the supply conductors l2 and I3 which areconnected to the secondary winding ll of a supply transformer, of whichthe primary winding (not shown) is connected to a suitablealternating-current source (not shown). Alternatively, the supplyconductors l2 and I3 may be directly connected to an alternating-currentsource of suitable voltage, e. g. 230 volts.

Although the electric valve ll may be of any suitable type, it ispreferably of the type having an anode, a cathode, and a control grid,and into the envelope of which a small quantity of an inert gas, such asmercury vapor or argon, is introduced. The presence of this gas withinthe envelope converts the usual pure electronic discharge into an arcstream, so that the valve becomes a grid controlled arc rectifier. Thegrid has control only of the starting of the discharge. However, byvarying the firing point, i. e., the tim in the positive half-cycle ofthe anode voltage at which the grid voltage attains the critical valuenecessary to initiate current flow in the anode-cathode circuit, theaverage value of the current which flows in the output circuit can bevaried. If the firing point occurs at or near the beginning of thepositive half-cycle of anode voltage, maximum current will how in theoutput circuit. If the firing point occurs at the degree point, thecurrent flow will be a minimum or zero. For intermediate firing pointsthe output current will have corresponding intermediate values.

As shown in the drawing, one armature terminal of the motor I0 isconnected to the supply conductor l2 and the other terminal is connectedto the anode Ha of valve II. The cathode llb of the valve is connectedto the opposite supply conductor l3.

Preferably, the field winding I00, of the motor is separately excitedfrom a suitable source, such as the double diode rectifier valve l5 andthe transformer secondary winding Hi to which it is connected forfull-wave rectification. The primary winding (not shown) of thistransformer may be supplied from any suitable source of alternatingvoltage.

For the purpose of varying the firing point of the valve H in eachpositive half-cycle of anode voltage, a network is provided whichcomprises the two electric valves l1 and I8 and. the capacitor IQ ofwhich the lower terminal Illa is connected to the supply conductor l3,and the upper terminal l9b is connected through a protective resistor 20to the grid llc of valv II. The terminal 19b is also connected throughthe ad- J'ustable resistor 2I, fixed resistor 22, and valve I 'I to theopposite supply conductor I2. Thus, the capacitor I9 is connected acrossthe cathode I lb and the grid I Ic of valve II, so that any voltageacross the capacitor will be applied across cathode I lb and grid I I0.

Preferably, the valve I1 is a diode valve having an anode I10 and acathode "D. The electric valve I 8 is a controlled valve having an anodeI8a, a cathode I8b, and a control grid I8c. This valve is connected in acircuit which extends from the supply conductor I3 through capacitor I9and motor armature I to the opposite supply conductor I2. It is thusconnected in parallel with diode valve I1 and is reversely connectedwith respect thereto, i. e., its anode I Be is connected to thecapacitor terminal I9b to which the cathode IIb of the diode isindirectly connected, and its cathode I8b is connected through armatureIll to the supply conductor I2 to which the anode Ila of the diode isconnected,

Owing to the reverse connection of the two valves I1 and I8, both halvesof the alternating voltage wave of the source I2, I3 are applied to thecapacitor I9 and thus the capacitor is alternately charged with voltagesof opposite polarity. Thus during the half-cycle in which the valve IIconducts, the voltage of the terminal I9!) is positive, and during thesucceeding half-cycle in which the valve I8 conducts, the capacitor isdischarged and recharged to the opposite polarity, so that the terminalI'9b has a negative voltage. If valves I1 and I8 conduct equal amounts,the voltage across the capacitor I9 is an alternating voltage having nodirect-current component. This alternating voltage will lag the anodevoltage of valve I I by approximately 90 degrees if the reactance ofcapacitor I 9 is small compared to the equivalent resistance of thevalves I I and I8. However, if the amounts of current conducted by thetwo reversely connected control valves are unequal, the alternatingvoltage across capacitor l9 will have a direct-current component, thepolarity of which depends upon which of the two oppositely directedcurrents is the greater and the magnitude of which depends upon thedifference in the amounts of the two currents. Thus, if the valve IIconducts more current than valve I8, the polarity of the netdirect-current component across capacitor I9 will be such that theterminal I9!) is positive with respect to terminal I9a. Conversely, ifvalve I8 conducts more current than valve II conducts, the voltage ofterminal 19b is negative with respect to terminal I911. Since th grid IIc of the valve I I is connected to terminal I9b, the effect of varyingthe direct-current component of the voltage across capacitor I9 is tovary the firing point of valve II. Thus, as the direct-current componentincreases in a. direction to make terminal I9b more positive, the firingpoint of valve II is advanced and the amount of current supplied byvalve II to the armature I0 is correspondingly increased. Conversely, asthe direct-current component is varied in a direction to make terminalI9b more negative, the firing point of valve II is retarded and theamount of current supplied to the armature I8 is correspondinglydecreased.

The amount of current conducted by the valve I8 is controlled by thedifference of a reference voltage and a voltage which is dependent uponthe speed of the motor. In order to provide for presetting the operatingspeed of the motor at a desired value, preferably the reference voltageis made adjustable. As shown, the source of relerence voltage is apotentiometer 23 which is connected in parallel with the motor fieldwinding Illa. The voltage proportional to the motor speed is derivedfrom the countervoltage of the armature. In order that th difference ofthese two voltages may be applied to the cathode-grid circuit of controlvalve I8, the positive terminal of potentiometer 23 is connected to thepositive terminal of armature I 0 by means of conductor 24, the grid Iis connected to the slider 23a, and the cathode I8b is connected to thenegative terminal of armature III. For the purpose of filtering theripple voltage from the reference voltage, a capacitor 25 is connectedbetween the positive terminal of potentiometer 23 and the slider 23a.

A dial 23b calibrated in terms of motor speed is operatively associatedwith the slider 23a,

A starting switch 25a is included in the armature circuit of the motorbetween the anod Ila and the point at which the connection to thecathode I8b is taken off. This is preferably an electromagneticallyoperated contractor, the operation of which is con-trolled by suitableswitching devices, such as pushbutton type starting and stoppingswitches. However, since the electromagnetic control circuits arconventional the contractor is illustrated in the drawing simply as amanually operated switch.

With the foregoing understanding of the elements and their organization,the operation of the System will readily be understood from thefollowing detailed description.

The speed at which it is desired to operate the motor is preset byadjusting the slider 23a to the point on the potentiometer 23 whichcorresponds to the desired speed as indicated by the calibrated dial.Since the motor is at rest and is not generating any countervoltage, thevoltage of the cathode I8b is more positive than the grid voltage, andthe current conducted by the valve I8 is zero or a minimum,Consequently, the direct-current component across capacitor I9 ismaximum in the direction which makes the terminal I9b and the grid I I0of valve I I more positive. As a result, the firing point of valve II isfully advanced.

The motor is started by closing the switch 25a to complete the armaturecircuit between the negative brush of the armature and the anode II a ofthe valve II, Since the valve I I is phased full on, it will supplymaximum current to the armature of the motor III. This causes the motorto accelerate at a rate which is dependent upon the load on the motorshaft. As the speed of the motor increases, the countervoltage generatedin the armature increases correspondingly, with the result that thevoltage at the cathode lb of valve I8 becomes increasingly negative asthe speed of the motor increases.

As a result of the voltage of the cathode I8b becoming negative, thevalve I8 becomes conducting during those half-cycles of thealternating-current voltage wave in which the valve II is nonconducting.The current conducted by the valve I8 reduces the net direct-currentcomponent across the capacitor I9 and may even reverse the polarity ofthis component, depending upon the position of the slider 23:: on thepotentiometer 23. The decrease of the net direct-current componentacross the capacitor I 9 from a positive value, or the increase of thiscomponent in a negative direction makes the voltage of the grid IIc morenegative with respect to its cathode III), The result of this is todecrease the current supplied by the valve II to the armature I0.

This acceleration of the motor I continues until a balanced orequilibrium condition is reached such that any further increase in thecountervoltage of the motor would cause the valve I I to decrease theamount of current supplied to the armature III to such an extent thatthe speed of the motor III would begin to decrease.

Owing to the connection of the cathode I8b of valve I8 to the anode IIn. of valve I I and the connection of anode lBa of valve I8 to the gridIlc of valve I I, the control valve I8 does not conduct an appreciableamount of current during the conducting period of the valve II.Consequently, the valve I3 is responsive to the voltage of the armatureI0 during the time in which no current is flowing in the armaturecircuit, and hence, during a time in which the countervoltage of thearmature is a true measure of the speed of the motor. Consequently, thecontrol is not affected by the RI drop of the motor armature, and thusthe control maintains the speed of the motor constant irrespective ofthe load.

A certain degree of compounding is obtained owing to the maintenance ofthe current flow through the valve I I for a portion of the negativehalf-cycle of the voltage wave as a result of the inductance of thearmature winding. In some cases, this current flow may be maintainedthroughout the first quarter of the inverse halfcycle. The portion ofthe inverse half-cycle of the voltage wave during which current flow ismaintained by the inductance of the armature is proportional to theamount of current which flows in the armature circuit during thepositive half-cycle. The result is that the total conducting time of thevalve I8 is proportionately reduced, which produces a proportionalturn-on tendency of the valve II. Thus, the control is slightlyovercompounded or compensated for the load. This results in holding thespeed of the motor very constant throughout the entire load range.

The system shown in Fig. 2 differs from the system of Fig. 1 in theprovision of a different source of reference voltage for presenting thespeed of the motor. In this modification, the conductors 25 and 21constitute an alternatingcurrent source corresponding to the conductorsI2, I3 of Fig. 1. The capacitor 28, the adjustable resistor 29, fixedresistor 30, and diode valve 3| correspond to the capacitor I9,adjustable resistor 2I, fixed resistor 22, and diode valve I1 of Fig. 1.The terminal 28a. of the capacitor 28 is connected to the conductor 21and the terminal 28b is connected both to the grid of the valve whichsupplies current to the armature of the motor and to the anode of thetriode just as in the modification of Fig. 1. This is not shown in Fig.2 but is indicated by appropriate legend.

A capacitor 32 is included between the anode of the diode 3I and theconductor 26 of the source. This capacitor 32 is charged to the polarityindicated and the voltage across the capacitor 32 is applied through asmoothing reactor 33 and resistor 34 to the terminals of a potentiometer35, one terminal of which is connected to the supply conductor 26. Theslider 35a of this potentiometer is connected to the grid of the triodecontrol valve (not shown) which corresponds to the valve I8 of Fig. 1and which is connected in the circuit in the same manner as the valve I8of Fig. 1. A gaseous type electric discharge diode valve 36 known to theart as a glow tube is connected across the potentiometer 35. Animportant characteristic of this type of valve is that the voltageacross its anode and cathode remains constant even though the voltage ofthe source varies, and consequently, the valve 36 serves as a voltageregulating tube for the voltage applied to the potentiometer 35. i

The desired operating speed of the motor is preset by moving the slider35a to the proper position on the potentiometer 35 as indicated on thecalibrated dial. In all other respects the operation is the same as theoperation of the system of Fig. 1 as described in the foregoing.

In the modification of Fig. 3, the capacitor 36a, the inductance 31, theresistor 38, the potentiometer 39, and the voltage regulating valve 40correspond to the capacitor 32, the inductance 33, the resistor 34,potentiometer 35, and voltage regulating valve 38 of Fig. 2, and haveidentical functions. The only difference between the modification ofFig. 3 and the modification of Fig. 2 is that the capacitor 36a is notcharged by current flowing through a diode valve which is connected tothe grid circuit of the valve which supplies current to the armature ofthe motor, but is charged by current conducted by an entirely separatediode valve M with which it is connected in series across thealternating-cur rent source 42, 420.

In the modification of Fig. 4, the capacitor 43, the inductance 44, theresistor 45, the potentiometer 46, the glow tube 41 and the diode valve48 correspond in structure and function to the capacitor 36a, inductance31, resistor 38, potentiometer 39, glow tube 40, and diode valve 4I,respectively, of Fig. 3. The only difference between the system of themodification of Fig. 4 and the system of the modification of Fig. 3 isthat the capacitor 43 instead of being charged by a halfwave rectifiervalve is charged by the full-wave diode rectifier valve 48 which issupplied from the secondary winding 49 of a transformer to the oppositeterminals of which the anodes 48a and 48b are connected as shown. Theprimary winding of the transformer is supplied from a suitablealternating-current source such as represented by the two conductors 50and SI. lar to the operation of the system of Fig. 1 as described in theforegoing.

In the modification of Fig. 5, the motor armature 52, the electric valve53, and the triode control valve 54 correspond to the armature II], thevalve II, and the triode control valve I8, respectively, of Fig. 1. Thearmature 52 is connected in circuit with the anode-cathode conductingpath of the valve 53 across an alternating-current source which isrepresented by the two supply conductors 55, 56. The difference betweenthe system of the modification of Fig. 5 and the system of themodification of Fig. 1 is that the reference voltage with which thevoltage proportional to speed is compared is provided by means of asource of reference voltage in the cathode circuit of the triode controlvalve. This source is illustrated as comprising a battery 51, and anadjustable potentiometer 58 connected across the battery 51. The grid54a of the triode control valve 54 is connected through a protectiveresistor 59 to the side of the source 55 with which the positiveterminal of the motor armature is connected.

The speed at which it is desired to operate the motor is preset byadjusting the slider 58a of the potentiometer to the point on thepotentiometer corresponding to the desired speed as indicated on thegraduated dial 60. The remainder of the operation is the same as theoperation of The operation is simithe system ofFig. 1 as described inthe foregoing.

The modified system of Fig. 6 is very similar to the system of Fig. 1.It differs from the system of Fig. 1 primarily in the use of a triodevalve in the phase shifting network instead of the diode valve ll ofFig. 1. Thus, in Fig. 6, the motor BI is connected across the A-C,source 62, 63 in series with a thyratron valve 64 which corresponds tothe thyratron valve H of Fig. 1. The capacitor 65, resistor 66 andtriode. valve 61 correspond to the capacitor l9, resistor 20 and triodevalve l8, respectively, of Fig. 1. Valve 68, which corresponds to valvell of Fig. 1, is a triode, and its grid is connected through a resistor69 to the anode of valve 61. A resistor III is connected across the gridand cathode of valve 68 and is included in the anode circuit of valve 61so that a voltage drop proportional to the anode current of valve 6! isproduced across its terminals. A capacitor H connected in parallel withthe resistor 10 is charged by this voltage drop and applies a biasvoltage to the grid of valve'68 which renders the grid of valve 68increasingly negative as the current transmitted by valve 61 increases.

A reference voltage is obtained from any suitable source such as theadjustable battery 12 which corresponds to the potentiometer 23 ofFig. 1. The difference between the reference voltage and thecountervoltage of motor 6| is applied between the cathode and grid ofvalve 81.

The operation is generally similar to the operation of the modificationof Fig. 1 and will readily be understood from the foregoing.

Although in accordance with the provisions of the patent statutes thisinvention is described as embodied in concrete form and the principlethereof has been explained, together with the best mode in which it isnow contemplated applying that principle, it will be understood thatalterations and modifications will readily suggest themselves to personsskilled in the art with-' out departing from the true spirit of thisinvention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A control system for a D.-C. electric motor comprising incombination, a source of alternating voltage, means for supplyinghalf-wave unidirectional current to the armature of said motorcomprising an electric valve provided with anode, cathode, and controlgrid and having its anodecathode path included in a circuit with saidsource and said armature, means for supplying to said grid a voltagehaving a periodically varying component and a direct componentcomprising a capacitor having connections to said grid and cathode and apair of reversely connected electric valves connected in circuit withsaid capacitor across said source, one of said reversely connectedvalves having an anode, a cathode, and a control grid, a source ofreference voltage, and connections for supplying to the grid and cathodeof said reversely connected valve the difference between a voltagederived from the armature terminals of said motor and said referencevoltage.

2. A control system for a D.-C. electric motor comprising incombination, means for supplying half-wave unidirectional current to thearmature of said motor comprising an electric valve provided with acathode, a control grid, and an anode connected to one terminal of saidarmature and connections from the other terminal of said armature andfrom said cathode to Opposite sides of a source of alternating voltage,and means for Supplying to said grid a periodically varying voltagehaving a direct-current component comprising a capacitor having oneterminal connected to said grid and the other to said cathode, anelectric valve connected in circuit with said capacitor across saidsource and a third electric valve connected from said grid to said anodeand provided with a. control grid, 9, source of reference voltage andmeans for applying to the grid and cathode of said third valve thedifference between said reference voltage and a votage derived from thearmature voltage of said motor comprising connections from Said sourceof reference voltage to the armature circuit of said motor and to thegrid of said second valve thereby to vary said direct-current componentto maintain the speed of said motor in correspondence with the magnitudeof said refence voltage.

3. A control system for a D.-C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor comprising a valve provided with an anode, acathode, and a control grid and connections from said anode to oneterminal of said armature and from the other terminal of said armatureand said cathode to opposite sides of said source, a source ofadjustable reference voltage having a connection to the armature circuitof said motor, and means for controlling said electric valve to causethe speed of said motor to correspond to the magnitude of said referencevoltage comprising a capacitor having one terminal connected to saidgrid and the other terminal connected to the same side of said source assaid cathode, a half-wave rectifier connected between the other side ofsaid source and said capacitor and a reversely connected rectifierhaving a cathode connected to said anode and an anode connected to saidgrid, and a grid connected to said source of reference voltage so thatthe difference of said reference voltage and a voltage proportional tothe speed of said motor is applied to the grid and cathode of saidreversely connected rectifier only during the inactive halfcycle of saidelectric valve.

4. A control system for a D.-C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor from an alternating-current source comprising anelectric valve having an anode and a cathode connected in circuit withsaid armature and provided with a control grid, 8. source of referencevoltage, means for producing a control voltage proportional to the speedof said motor, means for controlling the grid voltage of said firstelectric valve to cause the speed of said motor to correspond to themagnitude of said reference voltage comprising a second electric valveprovided with an anode, a cathode, anda control grid connected to beresponsive to the difference of said reference and control voltages, andconnections from said first valve to said second valve for renderingsaid second valve nonconducting when said first valve is conductingthereby to compensate the speed of said motor in accordance with theload.

5. A control system for a D.-C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor comprislng a first electric valve provided with acontrol grid and with an anode and cathode connected in circuit withsaid armature to a source of alternating voltage, means for supplying tothe grid of said valve a periodically varying voltage havingadirect-current component comprising a capacitor and two reverselyconnected electric valves connected in circuit with said capacitoracross said source, one of said reversely connected valves being atriode valve provided with an anode, a cathode, and a control grid, asource of adjustable reference voltage, means for producing a controlvoltage proportional to the speed of said motor, connections forsupplying to the cathode and grid of said triode valve the difierence ofsaid reference and control voltages to vary said direct-currentcomponent to cause the speed of said motor to correspond to themagnitude of said reference voltage, and connections from said firstelectric valve to said triode valve for rendering said triode valvenonconducting when said first electric valve is conducting thereby tocompensate the voltage supplied to said motor in accordance with theload.

6. A control system for a D. C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor comprising supply source connections, and a firstelectric valve provided with a control grid and with an anode and acathode connected in circuit with said armature across said supplyconnections, means for supplying to said grid a periodically varyingvoltage having a directcurrent component comprising a capacitor and tworeversely connected electric valves connected across said supplyconnections and connections from said capacitor to said cathode andgrid, one of said reversely connected valves being a triode having ananode, a cathode, and a control grid, a source of adjustabledirect-current reference voltage, and connections for supplying to thegrid and cathode of said triode valve the difference of said referencevoltage and a voltage derived from the armature of said motor thereby tovary said direct-current component to control said first electric valveto maintain the speed of said motor at a value corresponding to themagnitude of said reference voltage.

'7. A control system for a D. C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor comprising supply source connections, and a firstelectric valve provided with a control grid and with an anode and acathode connected in circuit with said armature across said supplyconnections, means for supplying to said grid a periodically varyingvoltage having a direct-current component comprising a capacitor and tworeversely connected electric valves connected across said supplyconnections and connections from said capacitor to said cathode andgrid, one of said reversely connected valves bea ing a triode having ananode, a cathode, and a control grid, a source of adjustabledirect-currentreference voltage comprising means for supplying adirect-current voltage to the field winding of said motor and apotentiometer connectedacross said field winding, and connections for Isupplying to the cathode and grid of said triode valve the difference ofa voltage derived from said potentiometer and a voltage derived from thearmature of said motor thereby to vary said direct-current component tocontrol said first electric valve to maintain the speed of said motor ata value corresponding to the magnitude of said reference voltage.

8. A control system for a D. C. electric motor comprising incombination, means for supplying half-wave current impulses to thearmature of said motor comprising supply source connections, and a firstelectric valve provided with a control grid and with an anode and acathode connected in circuit with said armature across said supplyconnections, means for supplying to said grid a periodically varyingvoltage having a directcurrent component comprising a capacitor and tworeversely connected electric valves connected across said supplyconnections and connections from said capacitor to said cathode andgrid, one of said reversely connected valves being a triode having ananode, a cathode, and a control grid, 9. source of adjustabledirect-current reference voltage comprising a rectifier and a secondcapacitor connected in series relationship to said supply connections,an adjustable potentiometer connected in a circuit in parallel to saidsecond capacitor and a voltage regulating valve connected in parallelwith said potentiometer for maintaining a substantially constant voltageacross said potentiometer, and connections for supplying to the cathodeand grid of said triode valve the difference of a reference voltagederived from said potentiometer and a voltage derived from said armaturethereby to vary said directcurrent component to control said firstelectric valve to maintain the speed of said motor at a valuecorresponding to the magnitude of said reference voltage.

9. A control system for a D. 0. electric motor comprising incombination, means for suplying half-wave current impulses to thearmature of an electric motor comprising supply source connections and afirst electric valve provided with a control grid and with an anode anda cathode connected in circuit with said armature across said supplyconnections, means for supplying to said grid a periodically varyingvoltage having a direct-current component comprising a capacitor and apair of parallel and reversely connected rectifiers connected in circuitwith said capacitor across said supply connections and connections fromsaid capacitor to said cathode and grid, one of said reversely connectedvalves being a triode and having an-anode, a cathode, and a controlgrid, a source of adjustable direct-current reference voltage comprisinga second capacitor connected in series with one of said reverselyconnected valves, a potentiometer connected in parallel with said secondcapacitor and a voltage regulating valve connected in parallel formaintaining a substantially constant voltage across said potentiometer,and connections for supplying to the cathode and grid of said triodevalve the difference of a reference voltage derived from saidpotentiometer and a voltage derived from said armature thereby to varysaid direct-current component to control said first electric valve tomaintain the speed of said motor at a value corresponding to themagnitude of said reference

